1. Industrial Field of Invention
The present invention relates to a semiconductor device which is capable of taking out an optical signal in the form of an electric signal. The semiconductor device according to the present invention is useful as a solid image pick-up device (also known as an xe2x80x9cimage sensorxe2x80x9d).
2. Prior Art
With the spread of facsimiles, more compact and lighter image sensors are demanded at a still lower cost. Image sensors used for facsimiles can be roughly classified into three types, i.e., a non-contact type, a contact type, and a complete contact type.
In non-contact type image sensors, the original manuscript is projected to a charge coupled device (CCD) through a reduction lens. The image sensors of this type are advantageous in that they can be readily fabricated by a well established LSI process using silicon wafers. Accordingly, they can be obtained at a low cost and with high productivity. However, on the other hand, the accompanying optics including the reduction lens and the like increases problems in the size and the weight of the entire sensor.
In contrast to the above non-contact type image sensors, those of the contact-type and the complete contact type are advantageous in that they can be fabricated into compact and light-weight devices. However, the production cost concerning the fabrication process, mounting, and assembling is disadvantageously high. Moreover, the image sensors of these types require the use of expensive components such as SELFOC (registered trade mark) lens arrays and thin glass sheets. Accordingly, the problem of costly production remains as a problem yet to be solved.
The contact-type image sensors utilized in facsimiles principally include those of multi-chip type comprising a plurality of MOSLSIs, and those of thin film type comprising optical sensor portions using thin film amorphous silicon and the like. At any rate, both of them use SELFOC lens arrays.
The image sensors of the multi-chip type can be obtained by a well established advanced technology based on the MOSLSIs fabrication process. Thus, it is believed that the image sensors of this type can provide a stable supply with a high product yield. However, because of the poor assembly precision, the packaged MOSLSI chips are not always uniform in characteristics.
In contrast to the image sensors of the multi-chip type, those of the thin film type can be readily formed in a large area because they can be fabricated by a thin film process on an insulating substrate made from a glass, a ceramics, and the like. Furthermore, reader chips for reading the original manuscript can be fabricated with the same length as the width of the original one. However, the chips of this type are disadvantageous in that they require a plurality of process steps, lower the product yield and hence increase the production cost.
Known photoelectric transfer elements include those of photo-conductor type and photo-diode type.
The photoelectric transfer elements of the photo-conductor type is made from a material whose resistance decreases upon receiving a light. Specifically mentioned as such a material include amorphous silicon. The change in resistance with the irradiation of light is read out as the change in electric current. In general, the photoelectric transfer elements of the photo-conductor type can afford a large flow of current and hence are resistant to noises. However, they suffer a poor optical response and are therefore not suitable for facsimiles in which high speed response is keenly demanded.
On the other hand, the photoelectric transfer elements of the photo-diode type produces a current signal at an intensity proportional to the intensity of light irradiated to the diode. More specifically, the carriers that are generated in the depletion layer by the light irradiated to the diode are swept with a reverse bias voltage. The elements of the photo-diode type quickly responds to light. However, the current flow in the photo-diode is so small that it is apt to be affected by noises.
Moreover, the photoelectric transfer elements of the photo-diode type above requires a plurality of complexed fabrication processes separately from fabricating a driver circuit for reading out the signal. The fabrication processes hence become complicated and includes a large number of steps. Hence, in general, the product yield tends to be lowered. Moreover, in case the driver circuit for reading out the signal is constructed by a separately added IC, since it requires a plurality of reader chips, it is difficult to lower the cost.
The aforementioned problems concerning image sensors can be overcome by a constitution satisfying the following requirements:
(1) a fabrication process composed of less process steps; the products can be fabricated at a high yield and hence at a low cost;
(2) a driver circuit for reading out the signal being assembled in the same substrate as that on which the photoelectric transfer element is provided; a plurality of external ICs can be hence excluded, and the sensor can be readily made in a large area at a reduced cost; additionally, a voluminous reduction optics is also omitted;
(3) a favorable optical response is assured, and is capable of high speed operation; and
(4) an intense signal can be obtained without being influenced by a noise.
An object of the present invention is to provide a photoelectric transfer element satisfying the above-mentioned requirements in items (1) to (4).
The object of the present invention above can be accomplished in one aspect by a semiconductor device illustrated in FIG. 1. The semiconductor device comprises a substrate 1 made of an insulating material such as glass or quartz. The semiconductor device amplifies the light signal irradiated to the device to be outputted as an electric signal corresponding thereto.
Referring to FIG. 1, the semiconductor device comprises an insulating substrate 1 having thereon an insulating layer 2, a crystalline silicon active layer 4 which generates photo carriers and which provides the channel forming region, a gate insulating film 6, a gate electrode 7, an interlayer insulating film 8, interconnecting electrodes 9 and 10, and one-conductive type source and drain regions 3 and 5 provided in contact with the active layer.
A predetermined voltage is applied to the gate electrode 7. In case source and drain regions 3 and 5 are N-conductive, a positive voltage is applied. If source and drain regions 3 and 5 are P-conductive, a negative voltage is applied.
As illustrated in FIG. 1, the key in the semiconductor device according to the present invention is to establish a structure comprising a crystalline silicon layer 4 which generates the photo carriers with the insulating layer 2 and the gate insulating film 6 sandwiching the crystalline silicon layer. The carriers generated by the irradiation of light are essentially required to be thus confined in the crystalline silicon layer 4. That is, a part of the thus generated photo carriers having the opposite polarity with respect to that of the carriers flowing in the active layer in the vicinity of the interface with the gate insulating film is temporarily accumulated within the active layer to change resistance of the active layer.
The operation principle of the semiconductor device is described below with reference to the constitution shown in FIG. 1. A light irradiated to said active layer is detected from the change in current flow between the source and the drain which occurs in accordance with the change in the resistance of the active layer. The semiconductor device outputs the optical signal irradiated to the crystalline silicon active layer 4 as an electric signal current flown between the source and drain regions.
The description below refers to a semiconductor device comprising an N-conductive semiconductor for the source and drain regions 3 and 5. A positive voltage is applied to the gate electrode 7. And an appropriate voltage is assumed to be applied between the source and drain.
In the above state, a case in which a light is irradiated to the active layer 4 is considered. Referring to FIG. 1, carriers are generated by photo-excitation, and the electrons, which are negatively charged, are attracted to the positively charged side of gate electrode 7. The holes having the positive charge move reversely towards the side of insulating layer 2.
The region to which the electrons are concentrated is reversed by the positive voltage applied to the gate electrode. Thus is formed a channel region having a low resistance. Hence, the electrons attracted to the gate electrode side contribute to the current flowing between the source and drain regions 3 and 5.
The holes concentrated to the side of the insulating layer 2 can not dissipate into the substrate by diffusion because an insulating layer 2 is provided on the substrate. Considering a case of an ordinary IC fabricated on a silicon wafer, this concept of the present invention corresponds to a state in which a substrate bias is applied (FIG. 2).
Referring to a MIS (metal insulator semiconductor) transistor fabricated on the surface of a single crystal silicon wafer 21, a channel region 24 is provided between source and drain regions 23 and 25.
In a state at which a substrate bias is applied as shown in FIG. 2, the resistance of the channel 24 changes with the shift in threshold voltage. Thus, the resistance of the channel 24 can be changed by varying the applied substrate bias. On the other hand, in the constitution shown in FIG. 1, a similar effect can be obtained by changing the number (density) of holes concentrated to the side of the insulating layer 2 of the active layer 4.
Referring to the constitution illustrated in FIG. 1, the holes concentrated in the side of the insulating layer 2 are photo carriers generated by the irradiation of light. The density of the holes is determined by the wavelength and the quantity of light irradiated to the active layer 4. Thus, the resistance of the channel formed on the side of the gate insulating film 6 in the active layer 4 can be controlled by the light irradiated to the active layer 4.
It is confirmed by experimentation that a current change corresponding to about 104 to 105 times the photo carriers generated in the crystalline silicon active layer 4 by the light irradiated thereto can be output as a current flowing between the source and drain.